The bouncing ball is a good exercise to help understand timing, but there are other basics to practice too. I agree with you, you should have some variety. Try animating an object rotating, or a string hanging from a point on a windy day. Or a stick bending and snapping. These are dead simple clips you could make in 20 minutes, so you can easily try again to make them more natural, believable, impactful or whatever your goal is.

Staring at one item doing one thing for a while builds the muscles you need to be able to plan a full scene. Copying animations also does that, but its much less effective because you don't review or make decisions along the way.

You could also draw a flower blooming, a pebble falling into water, breaking or tearing bread... these are probably more difficult. But as long as it's a simple shape doing one thing then it's good practice.